Lead potassium niobate substrate member for surface acoustic wave applications

ABSTRACT

A lead potassium niobate substrate having a singly rotated cut of the Z axis cylinder with a crystallographic orientation defined by the Euler Angles Lambda=74.4°, Mu=90.0° and Theta=0.0°.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

BACKGROUND OF THE INVENTION

This invention relates to surface acoustic wave devices and to asubstrate of lead potassium niobate, Pb₂ KNb₅ O₁₅, (PKN) for usetherewith.

ST-cut quartz is often utilized as a piezoelectric substrate materialfor a wide variety of surface acoustic wave devices (SAW) such asfilters, delay time encoders, decoders, correlators and other signalprocessing devices. Unfortunately, ST-cut quartz possesses a lowpiezoelectric coupling constant and, therefore, is not suitable for usein SAW devices designed to have low insertion losses and broadbandwidths. As a consequence, a considerable research effort has evolvedin an attempt to find other materials for use as SAW substrates thatpossess a high piezoelectric coupling constant and are temperaturecompensated. In attempting to find such materials, it has beendiscovered that in order to be temperature compensated, a material oftenpossesses either a positive temperature coefficient of an elasticconstant or a negative coefficient of thermal expansion. That such aconcept is valid has been demonstrated by the results of recentcalculations of the SAW properties of berlinite (which has a positivetemperature coefficient of an elastic constant) and β-eucryptite (whichhas a negative coefficient of thermal expansion). These calculationsshowed that both materials are indeed temperature compensated and havelarger piezoelectric coupling constants than ST-cut quartz.

However, berlinite and β-eucryptite still fail to possess apiezoelectric constant as large as is desired for certain SAWapplications and, in addition, lack the low temperature coefficient oftime delay and small electromechanical power flow angle parameters alsodesired for SAW applications. In attempting to find still newermaterials which might prove useful and desirable for SAW applications,it was discovered that lead potassium niobate (PKN), which occurs in thetungsten bronze structure and belongs to the orthorhombic crystal classmm₂ (C_(2v)) is attractive for SAW applications. The most significantfeature of the particular material of this invention is that itspiezoelectric coupling constant is up to 12.6 times as large as that ofST-cut quartz. In addition, the diffraction spreading of PKN is lessthan that of an isotropic material, an attractive feature not shared byeither quartz or berlinte.

Calculations undertaken during the research effort have shown that aparticular orientation of PEN having the Z-axis in the plane of theplate and perpendicular to the direction of preparation provides a highpiezoelectric coupling constant 12.6 times as large as that of ST-cutquartz. The particular crystallographic orientation of this inventionfor a Z-axis cylinder orientation is defined by the Euler angle:Lambda=74.4°; Mu=90.0°; and Theta=0.0°.

Currently, lithium niobate (LiNbO₃) is used as a substrate material forsurface acoustic wave devices which require greater bandwidths (for agiven amount of insertion loss) than that obtainable with ST-cut quartz.But, because LiNbO₃ has a large sensitivity to temperature, bulky andcostly ovens are required for temperature control. This newcrystallographic orientation of lead potassium niobate will make itpossible to build SAW devices with far greater bandwidths than thatpossible with quartz, and with greater temperature stability than thatpossible with lithium niobate.

The most important feature of the crystallographic orientation of thisinvention is that its piezoelectric coupling coefficient is about 12.6times as large as that of ST-Cut quartz. This makes it possible to buildlow insertion-loss SAW devices with low temperature sensitivity andlarger bandwidths than those obtainable in the devices currently beingbuilt on ST-cut quartz.

SUMMARY OF THE INVENTION

In accordance with the general concept of this invention, it has beenfound that lead potassium niobate provides a desirable and efficientsubstrate material for surface acoustic wave (SAW) applications.Calculations of the SAW properties of the substrate material haveproduced an orientation forming a singly rotated Z-axis cylindercrystallographic orientation (the Z-axis lies in the plane of the plateand is perpendicular to the direction of propagation) that provides thesubstrate with a low temperature coefficient of time delay and a zeroelectromechanical power flow angle particularly suitable for SAWapplications. The advantages of using the substrate of this inventionare achieved by a crystallographic orientation which is defined by theEuler angles Lambda=74.4°, Mu=90.0° and Theta=0.0°.

Accordingly, the primary object of this invention is to provide a novelsubstrate material for use in surface acoustic wave applications.

Another object of this invention is to provide a substrate material forsurface acoustic wave applications that is characterized by a lowtemperature coefficient of time delay and a small electromechanicalpower flow angle.

Still another object of this invention is to provide a substratematerial for surface wave acoustic wave applications that possesses veryhigh piezoelectric coupling characteristics.

The above and still other objects and advantages of the presentinvention will become more readily apparent upon consideration of thefollowing detailed description thereof when viewed in conjunction withthe accompanying drawings.

DESCRIPTION OF THE DRAWINGS

In the drawings

FIG. 1 is an isometric view illustrating the substrate member of PKNcontemplated by this invention; and

FIG. 2 is a graphical illustration showing various properties for theZ-axis cylinder shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Pursuant to the above-defined objects, it has been found that a leadpotassium niobate crystalline substrate with a Z-axis cylindercrystallographic orientation defined by the Euler angles Lambda=74.4°,Mu=90.0° and Theta=0.0° is especially suitable for surface acoustic waveapplications and exhibits a piezoelectric coupling constant of about12.6 times as large as that of the previously known ST-cut quartzsubstrate.

Initial interst in PKN as a possible SAW substrate material wasstimulated by measurements of its bulk wave properties which showed itto have electromechanical coupling factors as large as 0.73 and oppositesigns for the temperature coefficients of the fundamental resonantfrequencies for various crystal-cut plates. This latter result suggestedthat temperature-compensated cuts could be found for intermediateorientations. Further impetus was provided by the results ofmeasurements of the temperature coefficients of the elastic constants ofPKN, some of which are positive. To investigate the SAW properties ofPKN, a theoretical computer model was used to calculate the SAWvelocity, the electromechanical power flow angle, an estimate of thepiezoelectric coupling, and the first-order temperature coefficient oftime delay for singly rotated standard crystallographic orientation. Asinput, this model required experimental values for the elastic,piezoelectric, and dielectric constants, their respective temperaturecoefficients, the density, and the coefficients of thermal expansion.Values for all but one of these necessary constants were obtained fromthe data of Regnault, Ph.D. dissertation, (The Pennsylvania StateUniversity, 1977) (unpublished). His measurements were made on small(4-5 mm) single-crystal samples of PKN which were obtained from largercracked boules grown from a melt by the conventional Czochralski method.The samples were homogeneous and free of internal strains, but contained180° (electrical) domains which adversely affected only the measurementof the dielectric constant; hence, for the SAW property calculations, avalue for that quantity was obtained from the data of Yamada, J. Appl.Phys. 46, 2894 (1975). It should be noted that the effect of thedielectric constants on SAW calculations is only second order incomparison to that of the elastic or piezoelectric constants.

A crystalline orientation considered attractive for SAW applications wasfound for a single rotation of the Z-axis cylinder. Plots of the SAWproperties of the Z-axis cylinder are shown in FIG. 2. FIG. 1illustrates the lead potassium niobate substrate 10 of this inventionwith crystallographic axes and the Euler angles of this invention. Thecurves of FIG. 2 show that there is an attractive independent SAWorientations for the Z-axis cylinder. The orientation is listed in TableI with values for the temperature coefficient of time delay (TCD),piezoelectric coupling constant (ΔV/V∞), and SAW velocity. Forcomparison, the ST cut of quartz, two SAW orientations of berlinite,AlPO₄, and two of thallium vanadium sulfide, Tl₃ VS₄, are included also.The electromechanical power flow angle and its slope are not illustratedin FIG. 2 since the angle is 0.0.

                                      TABLE I                                     __________________________________________________________________________                                   Pow. Slp. of                                                              TCD Flow Power ΔV/V∞                                                                  SAW                                           Euler Angles                                                                              (ppm/                                                                             Ang. θ                                                                       Flw Angle                                                                           ×                                                                            Velocity                       Material                                                                              Orientation                                                                          λ                                                                           μ                                                                              θ                                                                          °C.)                                                                       (deg.)                                                                             (δφ/δφ)                                                         10.sup.-2                                                                          (m/sec)                        __________________________________________________________________________    Lead                                                                          potassium                                                                             Z-axis cyl.                                                                          74.4 90  0  0.0 0.0  -0.268                                                                              0.73 2505                           niobate 74.4°                                                          (Pb.sub.2 KNb.sub.5 O.sub.15)                                                 Quartz                                                                        (SiO.sub.2)                                                                           ST cut 0    132.75                                                                            0  0.0 0.0  0.378 0.058                                                                              3158                           Berlinite                                                                             X-axis boule                                                                         0    80.4                                                                              0  0.0 0.0  0.901 0.245                                                                              2751                                   boule 80.4°                                                            Doubly                                                                        rotated                                                                              79.7 90  15.5                                                                             0.0 0.0  0.221 0.247                                                                              2758                           Thallium                                                                              (110)                                                                 vanadium                                                                              cut    -45  90  70 0.0 -17.0                                                                              . . . 1.0   900                           sulfide 70°                                                            (Tl.sub.3 VS.sub.4)                                                                   (110)  45   24  90 0.0 0.0  . . . 0.617                                                                              1010                                   cylinder                                                                      24°                                                            __________________________________________________________________________

As the data in Table I shows, this new orientation of PKN possesses thedesirable combination of a small TCD and a zero electromechanical powerflow angle. Most significant, however, is the fact that thepiezoelectric coupling constant of this orientation is about 12.6 timesas large as that of the ST cut of quartz. It is known that, for a givenamount of insertion loss, the maximum attainable fractional bandwidth ofa SAW device is proportional to the square root of the ΔV/V∞ couplingconstant; hence, the large coupling of PKN makes feasible thedevelopment of temperature-compensated SAW devices with up to four timesthe fractional bandwidth possible with ST-cut quartz, for an equalamount of insertion loss. Hence, it can be seen from FIG. 2 that thereis a series of orientation whose piezoelectric coupling constantsdecrease only slightly from 0.0099 as the Euler angle λ is varied plusor minus 15.6° from 90.0°.

Another attractive feature shown by Table I is that for the Z-axiscylinder 74.4° orientation, the slope of the electromechanical powerflow angle, ∂θ/∂θ, lies between 0.0 and -1.0. According to SAWdiffraction theory, the diffraction spreading for this orientation isless than that of an isotropic material. This is another distinctadvantage of PKN over either quartz or berlinite.

As is the case with any new material, precise determination of thecrystallographic orientations possessing the attractive combination ofSAW properties discussed above awaits the availability of abundantsupplies of larger single-crystal samples of PKN. Currently, a crackingproblem, which occurs during the growth process, limits single crystalsto dimensions of only several millimeters. Solution of this crackingproblem will make it possible to obtain a more reliable set of materialconstants, calculate the temperature-compensated orientations moreaccurately, and construct devices to experimentally verify thosecalculations. Despite the need for greater accuracy in the locations ofthe temperature-compensated orientations, however, the data presentedherein show clearly that the PKN substrate of this invention will makefeasible the development of temperature-stable low-insertion-loss SAWdevices with far broader bandwidths than that possible with quartz, andwithout the temperature control schemes necessary with lithium niobate.

While the invention has been described by reference to a particularembodiment, it should be understood that those modifications asencompassed within the scope of the appended claims are intended to beincluded herein.

What is claimed is:
 1. A lead potassium niobate crystalline substratemember particularly adapted for use in surface acoustic wave devicewhich is characterized by having an acoustic wave propagation surfacedefined by a plane that substantially coincides with the Euler anglesLambda=74.4°, Mu=90.0° and Theta=0.0°.